Project Summary In highly structured organs, cells are arranged in precise patterns with each cell occupying a defined position and performing a specific function. Thus, there is an intimate linkage between a cell?s position and its fate. The mechanisms that regulate position usually involve changes in cell affinities and movement of cells, whereas the processes of fate assignment involve the execution of specific programs of cellular differentiation. This raises the question of how at the molecular level the mechanisms that determine position are integrated with those that determine fate. This proposal uses the developing Drosophila ommatidium as a model system with which to study the linkage between cell position and fate. Ommatidia grow by recruiting cells to precise positions in their structure, and the cell fate specifications depends on the cells they contact. The Receptor Tyrosine Kinase and Notch signaling pathways play key roles in ommatidial development, and in this work we test whether these two pathways control both the recruitment and cell fate specifications. Specifically, we ask whether the recruitment and fate specification steps represent early and late aspects of the same signaling process.